Generally, a construction machine such as a hydraulic excavator is equipped with, for example, a working mechanism comprising a boom, an arm and a bucket at a front portion of a vehicle body for performing a ground excavating operation of earth and sand or the like, and bearing devices are provided between the boom and the arm in the working mechanism, between the arm and the bucket therein and the like for rotatably connecting both of them.
The bearing device of this type according to the conventional art comprises a tubular boss member provided in one of the boom, the arm and the bucket and having a bush fitting bore at an inner peripheral side thereof, a bush provided to be fitted in the bush fitting bore of the boss member, a connecting pin inserted through an inner peripheral side of the bush to rotatably connect the boss member to an opponent member, and dust seals provided between the boss member and the connecting pin to be positioned at both of axial end sides of the bush (Patent Literature 1: Japanese Patent Laid-Open No. 2006-90400 A).
In this case, the dust seal is disposed between an inner peripheral surface of the boss member and an outer peripheral surface of the connecting pin, and, for example, serves to suppress foreign objects (dusts, rain water and the like) from entering into a sliding surface between the connecting pin and the bush from an outside. Namely, the dust seal is composed of a tubular fitting portion fitted on the inner peripheral side of the boss member with an interference, a bush end surface abutting portion extending radially inward from an end portion of the tubular fitting portion to abut against an axial end surface of the bush in a state of surface contact, and a lip portion extending to be inclined obliquely inward in an opposite direction to the bush from a radial inner section of the bush end surface abutting portion and having a tip end side in sliding contact with the outer peripheral surface of the connecting pin.
The dust seal is firmly mounted to the boss member by inserting and fitting the tubular fitting portion on the inner peripheral side of the boss member, and the lip portion is resiliently in sliding contact with the outer peripheral surface of the connecting pin in this inserting and fitting state. This dust seal prevents the foreign objects from entering into the sliding portion between the bush and the connecting pin from an outside. In addition, the lip portion of the dust seal serves as a valve for uniformly filling lubricant such as grease inside the bush at the time of supplying the lubricant to the sliding portion.
In the aforementioned conventional art, the bush end surface abutting portion of the dust seal is abutted against the axial end surface of the bush. However, the bush end surface abutting portion is simply in contact with the end surface of the bush. Therefore, there are some cases where a part of the lubricant such as grease supplied between the bush and the connecting pin flows out (leaks) into a region between the bush end surface abutting portion of the dust seal and the end surface of the bush.
When the lubricant flows thus out to the bush end surface abutting portion side of the dust seal from a region between the bush and the connecting pin, this lubricant enters between the end surface of the bush and the bush end surface abutting portion and serves to press the entire dust seal toward an axial outside of the boss member (bush fitting bore) by a pressing force thereof. Therefore, the pressing force of the lubricant possibly causes the dust seal to leave away from the end surface of the bush and be pulled out to an opponent member side.
On the other hand, in the aforementioned conventional art according to Patent Literature 1, for preventing this pulling-out of the dust seal, a pulling-out preventive portion of a convex shape is provided at the outer peripheral side of the tubular fitting portion. However, when the pulling-out preventive portion of a convex shape is formed at the outer peripheral side of the tubular fitting portion, for example, extra labors are required for a molding operation and an extraction operation of the dust seal using a mold tool, raising a problem of degradation in operability upon molding and processing the dust seal. Further, since the fitting force becomes uneven caused by the pulling-out preventive portion of a convex shape, the sealing force is also possibly uneven.
Further, even in a case of adding such pulling-out preventive portion to the tubular fitting portion of the dust seal, the dust seal may be pushed and moved in a direction of leaving away from the end surface of the bush, that is, toward an opponent member side by a pressing force of the lubricant entering into the bush end surface abutting portion side, raising a problem that the pulling-out preventive effect of the dust seal cannot be sufficiently expected.